The present invention relates to a method for the preparation of an ion exchanger for cesium ions and a method for the regeneration of the ion exchanger having cesium ions adsorbed thereon. More particularly, the invention relates to a method for the preparation of an ion exchanger suitable for the process of ion exchange separation of cesium ions, for example, from an aqueous solution of radioactive waste materials as well as to a method for the regeneration of the ion exchanger after adsorption of cesium ions thereon.
As is known, one of the very important problems in the technology for the disposal of radioactive waste materials as well as for the utilization of radioactive materials is the technology for the separation and recovery of radioactive cesium ions in an aqueous solution such as those discharged in large volumes from various facilities for the utilization of atomic power including the plants for reprocessing of spent nuclear fuels.
Such an aqueous waste solution in general characteristically is strongly acidic with nitric acid or contains a large amount of sodium nitrate so that the method for the separation of radioactive cesium ions must comply with this unique condition of the aqueous waste solutions. Various methods therefor have been proposed heretofore including the precipitation method, solvent-extraction method, ion-exchange method and so on although none of these prior art methods is quite satisfactory as an industrial process. Some of the difficulties encountered in these prior art methods include increase in the amount of secondary waste materials and consumption of a large amount of chemical reagents not re-utilizable resulting in an undue increase in the salt concentration in the solution after treatment by the decomposition products as well as the adverse influences caused on the downstream processes by the complicated decomposition treatment undertaken for the removal of the salt in the solution under treatment. Taking the ion-exchange method as an example, one of the reasons in principle for these problems is the difficulty in the desorption and recovery of cesium ions selectively adsorbed on an ion exchanger in the presence of a large amount of electrolytes.
The inventor has previously proposed, in Japanese Patent Kokai 5-254828, that, by the use of a specific hexacyanoferrate (II) capable of selectively adsorbing cesium ions by ion exchange, an efficient method for the adsorption and desorption of cesium ions on and from such an ion exchanger of the oxidation-reduction type enabling repeated use thereof in view of the possibility to overcome the above mentioned difficulties by the utilization of the function for oxidation-reduction which an insoluble body of the ion exchanger possesses. This prior art method is particularly effective for the copper hexacyanoferrate (II) and is characterized in that cesium ions in an aqueous solution containing nitric acid in a high concentration can be efficiently adsorbed on to the ion exchanger in a reduced state (in the presence of an oxidation-preventing agent or nitrous acid remover in the former case) followed by the oxidation treatment of the ion exchanger with an aqueous nitric acid solution containing nitrous acid to effect efficient desorption of the cesium ions while the ion exchanger in an oxidized state can be regenerated by the treatment with a reducing agent such as a hydrazinium salt so that the ion exchanger can be used repeatedly with a remarkably small amount of waste materials to be disposed.
A presumable scheme in the above mentioned prior art method for the steps of adsorption of cesium ions on the ion exchanger, desorption of the cesium ions from the ion exchanger with nitric acid and regeneration of the ion exchanger with a reducing agent includes the following reaction equations (1), (2) and (3), respectively: EQU A.sub.2 Cu.sub.3 [Fe(CN).sub.6 ].sub.2 +2Cs.sup.+ .fwdarw.Cs.sub.2 Cu.sub.3 [Fe(CN).sub.6 ].sub.2 ; (1) EQU Cs.sub.2 Cu.sub.3 [Fe(CN).sub.6 ].sub.2 (+HNO.sub.3).fwdarw.Cu.sub.3 [Fe(CN).sub.6 ].sub.2 +2Cs.sup.+ ; (2)
and EQU Cu.sub.3 [Fe(CN).sub.6 ].sub.2 +2A.sup.+ (+reducing agent).fwdarw.A.sub.2 Cu.sub.3 [Fe(CN).sub.6 ].sub.2, (3)
in which A.sup.+ is a monovalent cation which can be a hydrogen ion or a cation such as an ammonium ion as a decomposition product from the hydrazinium salt as the reducing agent when reduction is performed in an acidic condition under a stationary state.
A problem to be solved in the above described prior art method is that the insoluble hexacyanoferrates in general can be obtained only in the form of a fine powder so that difficulties are encountered in the solid-liquid separation of the ion exchanger from the liquid medium. In this connection, a suggestive teaching is obtained from Journal of Nuclear Science and Technology, volume 4, No. 4, pages 190-194 (1967) which discloses an adsorbent for cesium ions prepared by the deposition of copper hexacyanoferrate (II) in the pores of a porous anion exchange resin. The inventor attempted a test for the applicability of the above described prior art method to this adsorbent which could assumably be an ion exchanger of the oxidation-reduction type only to find that adsorptivity of cesium ions in the first time is low as compared with the adsorptivity in the second time and later on and that elution of a considerably large amount of hexacyanoferrate ions takes place into the solution for the adsorption treatment and the solution obtained by the desorption treatment in the second time and later on resulting in depletion of the ion exchanger.